class Solution {
    int m, n, dx[4] = { 0, 0, 1, -1 }, dy[4] = { 1, -1, 0, 0 };
    void dfs(vector<vector<char>>& board, int i, int j)
    {
        board[i][j] = '.';
        for(int k = 0; k < 4; k++)
        {
            int x = i + dx[k], y = j + dy[k];
            if(x >= 0 && x < m && y >= 0 && y < n && board[x][y] == 'O')
                dfs(board, x, y);
        }
    }
public:
    void solve(vector<vector<char>>& board) {
        // 遍历边界
        // 将位于边界的"O"通过dfs修改为"."
        m = board.size(), n = board[0].size();
        for(int j = 0; j < n; j++)
        {
            if(board[0][j] == 'O')
                dfs(board, 0, j);
            if(board[m - 1][j] == 'O')
                dfs(board, m - 1, j);
        }

        for(int i = 0; i < m; i++)
        {
            if(board[i][0] == 'O')
                dfs(board, i, 0);
            if(board[i][n - 1] == 'O')
                dfs(board, i, n - 1);
        }

        // 还原
        for(int i = 0; i < m; i++)
        {
            for(int j = 0; j < n; j++)
            {
                if(board[i][j] == '.')
                    board[i][j] = 'O';
                else if(board[i][j] == 'O')
                    board[i][j] = 'X';
            }
        }
    }
};